1. Field of the Invention
The invention relates generally to the class of optical fibers made from single crystals such as sapphire and in particular to a new class consisting of graded index single crystal optical fibers. It also relates to methods of producing such fibers.
2. Invention Disclosure Statement
Generally in the prior art, single crystal optical fibers are clad with plastic materials. It has not been possible to prepare core/clad or graded index structures for single crystal fibers because the fibers must be grown from the melt. It is very difficult to maintain significant variations in concentrations of dopants in a liquidus region. By contrast glassy materials like silica are drawn into fibers from the softened glassy state. The viscosity of the softened glassy state is very high and restricts significant exchange of dopant between layers of the preform Under conventional methods, flow within the liquidus state of the material is thermal gravitational flow which tends to distribute materials, including dopants, randomly and somewhat uniformly throughout the liquidus region.
One solution to the problem has been to grow a single crystal optical fiber and then deposit a dopant layer on the outer surface of the fiber. The dopant is diffused into the fiber by maintaining the fiber at a high temperature for a period of time. Lithium niobate, LiNbO.sub.3, single crystal fibers have been doped with magnesium oxide, MgO, by this method. [See S. Sudo, I. Yokohama, A. Cordiva-Plaza, M. M. Fejer and R. L. Byer, Appl. Phys. Lett. 58, 1931 (1990).]
The process required multiple extended exposures to temperatures of about 1050.degree. C. including periods of 2 hours for annealing before depositing the dopant layer and for 40 hours to obtain diffusion to 10 .mu.m into a 90 .mu.m diameter lithium niobate fiber. Overall the process is slow, complex and involves many steps. There are, thus, many places for mistakes to arise, making the production of consistently doped fibers difficult, particularly in quantity. Fabrication costs are also expected to be high and difficult to reduce.
It has also been noted in the literature [R. S. Feigelson, W. L. Kway and R. K. Route, Opt. Eng. 24, 1102 (1985)] that a distribution of laser active dopants in YAG garnets [also single crystals] occurs as the doped single crystals are grown into fibers by the laser heated pedestal growth [LHPG] method. [See e.g., U.S. Pat. No. 4,421,721.] Preliminary micro distribution measurements indicated that samples made by different researchers had varying radial distributions. Note that dopant concentrations required for lasing fibers are not functional for normal graded-index fiber applications. Also the authors implied a uniform radial distribution would make for a good fiber laser.
One of the problems with the current processes to fabricate single crystal fibers is in providing a thermal zone with good axial symmetry in which a small pool of the material can be maintained in the liquid state. A second major problem, as described above, is the need to find a way to establish or maintain a variation in dopant concentration across the radial cross section of the fiber as it is grown from the melt. As noted earlier, alternative methods, which employ deposition of a dopant followed by its diffusion, have problems with complexity, consistency and very slow production rates. It is likely that fabrication costs would be too high for major commercialization.